An electronically controlled fuel injection valve is opened by a driving pulse signal (injection pulse) given synchronously with the rotation of an engine and while the valve is opened, a fuel is injected under a predetermined pressure.
Accordingly, the injection quantity of the fuel depends on the period of opening of the valve, that is, the injection pulse width. Assuming that this pulse width is expressed as Ti and is a control signal corresponding to the injection quantity of the fuel, Ti is expressed by the following equations: EQU Ti=Tp.times.COEF.times..alpha.+Ts and Tp=K.times.Q/N
wherein Tp stands for the injection pulse width corresponding to the basic injection quantity of the fuel, which is called "basic fuel injection quantity" for convenience, K stands for a constant, Q stands for the flow quantity of air sucked in the engine, N stands for the rotation speed of the engine, COEF stands for various correction coefficients for correcting the quantity of the fuel, which is expressed by the following formula: EQU COEF=1+Ktw+Kas+Kai+Kmr+Ketc
in which Ktw stands for a coefficient for increasing the quantity of the fuel as the water temperature is lower, Kas stands for a correction coefficient for increasing the quantity of the fuel at and after the start of the engine, Kai stands for a correction coefficient for increasing the quantity of the engine after a throttle valve arranged in an intake passage of the engine is opened, Kmr stands for a coefficient for correcting the air fuel mixture, and Ketc stands for other correction coefficient for increasing the quantity of the fuel, .alpha. stands for an air-fuel ratio feedback correction coefficient for the feedback control (.lambda. control), described hereinafter, of the air-fuel ratio of the air-fuel mixture, and Ts stands for the quantity of the voltage correction for correcting the change of the flow quantity of the fuel injected by the fuel injection valve, which is caused by the change of the voltage of a battery.
In short, the desired injection quantity of the fuel is obtained by multiplying the basic fuel injection quantity Tp by various correction coefficients COEF, and when a difference is brought about between the aimed value to be attained by the control and the actual controlled value, this difference is multiplied by .alpha. to effect the feedback control and the correction for the power source voltage is added to the feedback control.
The feedback control of the air-fuel ratio will now be described. An exhaust component concentration detecting member, for example, an O.sub.2 sensor for detecting the oxygen component in the exhaust gas, is attached to an exhaust passage to detect the actual air-fuel ratio .lambda. of the air-fuel mixture sucked in the engine, and by comparing with a slice level, it is judged whether the actual air-fuel ratio .lambda. is richer or leaner than the aimed air-fuel ratio .lambda.t. When a known ternary catalyst for efficiently converting CO, HC and NO.sub.x, the main three exhaust gas components, at the theoretical air-fuel ratio is arranged in the exhaust system, the above-mentioned aimed air-fuel ratio .lambda.t is equal to the theoretical air-fuel ratio. Accordingly, in this case, by the slice level, it is judged whether the actual air-fuel ratio is richer or leaner than the theoretical air-fuel ratio, and the injection fuel quantity expressed by Tp.times.COEF is increased or decreased and controlled so that the actual air-fuel ratio becomes equal to the theoretical air-fuel ratio. For this control, the air-fuel ratio feedback correction coefficient .alpha. is set and the injection quantity Tp.times.COEF is multiplied by .alpha..
If it is intended to effect the feedback correction at a time by abruptly changing the value of the air-fuel feedback correction coefficient .alpha., the theoretical air-fuel ratio is overshot or undershot, and therefore, the value of the air-fuel ratio feedback correction coefficient is changed by the proportion and integration (PI) control so that the air-fuel ratio is stably controlled.
More specifically, in the case where the output of the O.sub.2 sensor is higher or lower than the slice level, the air-fuel ratio is not abruptly leaned or riched, but in the case where the air-fuel ratio is rich (lean), the air-fuel ratio is first decreased (increased) only by the proportional (P) component, and is then gradually decreased (increased) by the integration (I) component unit so that the air-fuel ratio is leaned (riched). The P component is set at a value sufficiently larger than the I component unit.
In the region where the air-fuel ratio feedback control is not performed, the value of .alpha. is clamped to 1 or a constant value.
Needless to say, if the base air-fuel ratio in the region where the air-fuel ratio feedback control is effected, that is, the air-fuel ratio at the time when .alpha. is equal to 1, is set at the theoretical air-fuel ratio (.lambda.=1) through the entire region, the feedback control is inherently unnecessary. Practically, however, even if the base air-fuel ratio is set at .lambda.=1 in a specific driving state, the air-fuel ratio is ordinarily deviated from the theoretical air-fuel ratio in other driving state because of deviations or changes with the lapse of time among constituent members (such as an air flow meter, a fuel injection valve, a pressure regulator and a control unit), the non-linearity of the pulse width-flow amount characteristic of the fuel injection valve and changes of the driving conditions and environments. In this region where the deviation of the base air-fuel ratio occurs, the air-fuel ratio feedback control is performed so that this deviation is eliminated. This air-fuel ratio feedback correction control is disclosed in, for example, U.S. Pat. No. 4,284,050, U.S. Pat. No. 3,483,851 and U.S. Pat. No. 3,750,632.
However, in this air-fuel ratio feedback control, for example, when one stationary driving region is greatly changed to a different stationary driving region, if the base air-fuel ratio in this different stationary driving region is greatly deviated from .lambda.=1, it takes too long a time to perform the PI control of the change of the base air-fuel ratio generated by this deviation to .lambda.=1 by the feedback control. More specifically, even though the base air-fuel ratio has been obtained from the specific injection quantity Tp.times.COEF and the deviation of this air-fuel ratio from the theoretical air-fuel ratio has been corrected by the PI control based on .alpha., since the base air-fuel ratio is greatly changed, the base air-fuel ratio is controlled to a value greatly different from .lambda.=1 if Tp.times.COEF used up to this time is still used, and the feedback correction by similar PI control should be performed and it takes a long time to correct the base air-fuel ratio to .lambda.=1 by the feedback correction. In order to eliminate this disadvantage, it is necessary to improve the respondency of the control by increasing the PI constant. However, if the control respondency is thus improved, overshooting or undershooting is readily caused and the control performance is degraded. Namely, when the base air-fuel ratio is greatly deviated from .lambda.=1, the control of the air-fuel ratio is effected in the region separate greatly from the theoretical air-fuel ratio.
Consequently, the driving is carried out in the range where the conversion efficiency of the ternary catalyst is low, and therefore, increase of the cost by increase of the amount of the noble metal in the catalyst is caused and the catalyst should be exchanged with new one frequently because of further reduction of the conversion efficiency due to deterioration of the catalyst.
A control system in which the above-mentioned disadvantage is eliminated by learning the control quantity controlled by the system and increasing the respondency of the air-fuel ratio control in the same driving state has been proposed by us in Japanese Patent Application Laid-Open Specifications No. 203828/84 and No. 203829/74 and U.S. patent application Ser. No. 604,025.
According to this control system, learning control of the air-fuel ratio feedback control is first carried out. More specifically, in the air-fuel ratio feedback control region, if the base air-fuel ratio is deviated from the aimed air-fuel ratio .lambda.t, since the feedback correction coefficient .alpha. is increased for compensating this gap during the process of transfer, the driving state at this time and .alpha. are detected, and the learning correction coefficient .alpha.o based on this .alpha. is determined and stored. When the same driving state is brought about, the base air-fuel ratio is corrected to the aimed air-fuel ratio .lambda.t with a good respondency by the stored learning correction coefficient .alpha.o. Storing of the learning correction coefficient .alpha.o is performed for all of engine-driving state areas of a predetermined range formed by lattice division of a map of RAM according to the rotation speed of the engine and the engine-driving conditions such as the load.
More specifically, the map of the learning correction coefficient .alpha.o corresponding to the rotation speed of the engine and the driving conditions of the engine such as the load is formed on RAM, and when the injection quantity Ti is calculated, the basic injection quantity Tp is corrected by .alpha.o as shown by the following equation: EQU Ti=Tp.times.COEF.times..alpha..times..alpha.o+Ts (1)
Learning of .alpha.o is advanced according to the following procedures.
(i) The engine-driving state in the stationary state and the median .alpha.c of control of .alpha. (the mean value of a plurality of values .alpha.o at the time of reversion of increase of decrease of the output signal of the O.sub.2 sensor) are detected.
(ii) The value .alpha.o (old) heretofore learned, corresponding to the engine-driving state, is retrieved.
(iii) The value of .alpha.o(old)+.DELTA..alpha./M is determined from .alpha.c and .alpha.o(old), and the storage is renewed with the obtained value (learned value) being as new .alpha.o(new).
Incidentally, .DELTA..alpha. stands for the deviation from the standard value .alpha.1 and expressed by .DELTA..alpha.=.alpha.-.alpha.1. However, in order to take a mean value, .DELTA..alpha. is expressed by .DELTA..alpha.=.alpha.c-.alpha.1 and the standard value .alpha.1 is ordinarily set at 1.0 as the value corresponding to .lambda.=1. M is a constant.
According to this learning system in the conventional air-fuel ratio feedback control, a good detection precision of the deviation quantity .DELTA..alpha. is obtainable only in the stationary state, and therefore, only in the stationary state, learning is performed by detecting .DELTA..alpha.. Accordingly, learning is not performed in the area of the temporary driving state which passes in the transitional driving.
As the result, there are produced an area of a large degree of the advance of learning (hereinafter referred to as "learned area") and other area of a small degree of the advance of learning (hereinafter referred to as "unlearned area").
In the transitional stage between different engine-driving states, a step of the air-fuel ratio is produced between the learned area and the unlearned area or between two unlearned areas, and the exhaust emission in the transitional state is worsened and no substantial effect is attained.
It is therefore a primary object of the present invention to improve the control precision in the transitional driving stage by estimating the learning correction coefficient of the unlearned area from the area of other driving state of a large degree of the advance of learning and using the estimated learning correction coefficient .alpha.s.
Another object of the present invention is to obtain the above-mentioned estimated learning correction coefficient .alpha.s from the learning correction coefficient .alpha.o stored in the neighbouring learned area by interpolatory calculation.
It is considered that among factors causing the deviation from the base air-fuel ratio of .lambda.=1, those owing to changes of the characteristics of the fuel injection valve by adhesion of dusts, wearing and the like occupy large proportions. It also is considered that in the regions where the fuel injection quantity Tp is the same, the measurement error .DELTA.Tp of the fuel injection quantity Tp is similarly the same. Furthermore, it is considered that among the factors causing the deviation from the base air-fuel ratio of .lambda.=1, the measurement error of the flow quantity Q of intake air by the intake air flow quantity detecting means occupies a considerably large proportion, and for example, in case of a hot wire type air flow meter, the measurement error is prominently increased by adhesion of dusts to the hot wire or deterioration of the hot wire per se. In this case, it is considered that in the regions where the intake air flow quantity .DELTA.Q is the same, also the measurement error .DELTA.Q of Q is the same.
Therefore, another object of the present invention is to improve the reliability of the learned value and increase the precision of control of the air-fuel ratio by determining the estimated learning correction coefficient .alpha.s by estimating, based on the learned value .alpha.o(new) now obtained by learning, the learning correction coefficient .alpha.o in an area of the driving state of a small degree of the advance of learning where the fuel injection quantity Tp or the intake air flow quantity Q is the same as in the driving state area of said new learned value.